We review a number of highly charged ions which have optical transitions suitable for building extremely accurate atomic clocks. This includes ions from Hf 12+ to U 34+ , which have the 4f 12 configuration of valence electrons, the Ir 17+ ion, which has a hole in almost filled 4f subshell, the Ho 14+ , Cf 15+ , Es 17+ and Es 16+ ions. Clock transitions in most of these ions are sensitive to variation of the fine structure constant, α (α = e 2 /hc). E.g., californium and einsteinium ions have largest known sensitivity to α-variation while holmium ion looks as the most suitable ion for experimental study. We study the spectra of the ions and their features relevant to the use as frequency standards. Currenf microwave cesium clock, which serves as definition of metric second, have Q ∼ 10 16 [7], best optical clocks approach Q ∼ 10 18 [8-10] mostly due to larger frequency. Further progress can be achieved by using optical transitions in HCI [1,2]. HCI are less sensitive to perturbations due to thier compact size. Therefore, quality factor Q = ω/δω can be larger than in neutral atoms due to smaller δω. In this paper we review some recent proposals for very accurate atomic clocks based on HCI and thier use for the search of time variation of the fine structure constant.
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